This invention relates generally to a target for retro-reflective photogrammetry that provides for critical detection of light from the strobe of a close range industrial photogrammetry camera, and more specifically to a precisely machined aperture ring and cooperating socket that defines a target assembly that provides for precise area and location of the target to within +/−0.0005 inch or less.
Targets with closely located mask overlays, used in photogrammetry and automated theodolite systems, have been available for years in the prior art. Generally, of more current usage, the Global Positioning System, or GPS, is used indoors as a method to precisely set various industrial devices and industrial equipment, including machinery and for close tolerance industrial surveying, as used in manufacturing and assembly operations. Normally, infrared targets without a mask overlay have tolerances within several thousandths of an inch, which may be insufficient to obtain the precision required in a machining operation, in the manufacturing of parts in particular industries, or in industrial surveying. Additionally, laser projection systems emit a laser beam along a line that represents the position of an edge used in a step of manufacturing, such as the position of a composite lay up sheet.
For reference, retro-reflective photo targets, with a mask overlay have had countless design adaptations to enhance precision for the various settings and component features that are measured using close range industrial photogrammetry. These adaptations may include straight holes, threaded holes, surfaces, edges, slots and the like. The prior art frequently obtains tolerances as close as +/−0.0.005 inch. In achieving such tolerances, an adhesive masked overlay is located and inspected with a high degree of precision in the controlled environment of a factory. The factory provides the special tooling and fixtures, inspection masters and equipment, and the trained and highly skilled people that operate the tooling and conduct the inspections. Though tight tolerances are achieved, much skill, equipment, and time are expended in doing so.
Currently, most targets, for the photogrammetric methods and systems, generally use a 3M® type tape material that has small glass beads coated onto its top surface, with a reflective coating on the back side of the beads, and integral with the tape. When the strobe light from the photogrammetric camera fires, it causes the glass beads on the tape to reflect the light and thereby creates spots of light that appear on a photograph. The spots are then measured for three-dimensional locations in the X, Y and Z axes. Other targets, as used in photogrammetry, serve as reference points, without achieving a critical X, Y and Z location. These types of targets may be simply an adhesive strip upon a machine or item to be surveyed. Over time and with usage in field conditions, the retro-reflective tape wears or becomes damaged and its reflectivity diminishes. Diminished tape requires replacement. Much like initial assembly of a target, the tape and any overlays, upon close tolerance hard body targets, are replaced by highly skilled staff at a factory location. Replacing the tape and any overlays on critical targeting in the field jeopardizes the tolerances of any targets.
However, many targets require critical dimensions and are mounted upon a close tolerance steel body that supports the target, generally called hard body targets. The intention is for the retro-reflective target to be located precisely in three dimensions on the body which in turn usually represents an X, Y and Z value of the machine or item being positioned or surveyed. The 3M® retro-reflective material is adhered to a tape product of a known definite thickness, the material being covered with small glass beads of approximately 0.003 inch diameter. This material has a granular appearance similar to that of grit on sandpaper. However, in many inspection and quality control steps and operations, the tolerance of the target dot location must be +/−0.0005 inch or less. As a result, the components of the standard materials without a mask overlay make it impossible to meet the tolerances required.
Other prior art includes products stating a high precision of the target location but only on the centerline of the shank of the target to within 0.001 inch on concentricity or 0.0005 inch on centerline. The height tolerance for the target is 0.005 inch which does not meet the needs of industry. The present invention has a tolerance much tighter than the prior art.
To accomplish the final close tolerance that complies with job requirements, some industries use a mask overlay that is critically located in two directions, and a third critical location is the surface of the 3M® material that adheres to the tape and compensates for the thickness of the material used. The final application of the mask overlay produces an area of reflectivity that is controlled by size, roundness, crispness, clarity and critical location in relation to the target body.
As can be seen in the prior art and in U.S. Pat. No. 5,073,005, to Hubbs, obtaining greater precision through the usage of a mask overlay, applied over a target, and a mask that may have a reflective member applied under it, can attain precise locations, generally within +/−0.0005 inch tolerance or less. This occurs through the use of a mask that has a finite aperture that allows the entrance of the light from the camera strobe therethrough, and adds precision to the establishment of the X, Y and Z axes when such a surveying instrument, applying photogrammetry, is used.
The present invention uses a precisely machined ring that fits in a slip fit relationship into a precisely machined socket. The socket has an inside circular portion for retro-reflective tape over which fits the machined ring. The resulting target apparent through the ring achieves a tolerance of +/−0.0005 inch along centerline, for concentricity, and for height offset.
The goal of this invention is to produce a retro-reflective photogrammetry target that represents X, Y, Z values within a tolerance of +/−0.0005 where the target can be refurbished in field conditions. The present invention reaches this goal by using a ring and a cooperating socket that frame retro-reflective tape upon a target at precise tolerances. The ring shapes and locates the target precisely to a very critical location. The current design consists of multiple components, most having close tolerance assembly features. The difficulty of manufacturing multiple components with very close tolerances is that variations or tolerances stack up, or accumulate, during assembly of a target which jeopardizes the goal of a final target at a location within a 0.0005 inch or less tolerance. Further, the replacement of retro-reflective tape requires skilled labor and equipment in a factory setting unlike the prompt repairs often demanded in the field.